Electric devices, such as headlamps, tail lamps, fog lamps, power windows, pressure sensors, pressure switches, and engine control units of automobiles, are used in an environment in which they are exposed to liquid, such as water, oil, and surfactants. Further, also among general home appliances, electric razors, mobile phones, electric toothbrushes, and others are used in an environment in which they are exposed to liquid, such as water, oil, and surfactants. In these electric devices, a case for putting electronic components therein needs to be highly waterproof. If, however, the case is completely airtight, the expansion and contraction of gas in the case due to temperature changes causes a difference in pressure between the inside and outside of the case, and excessively encumbers the case. Accordingly, the case needs to remain capable of allowing gas to enter and exit from it, while preventing liquid from entering.
Patent Document 1 describes a vent cap composed of a cover component having a cylinder shape with a bottom, and a cylindrical body fitting within the cover component, wherein a labyrinthine vent passage is formed between the inner periphery of the cover component and the outer periphery of the cylindrical body, and also between the bottom surface of the cover component and the bottom portion of the cylindrical body, so that the vent cap exhibits waterproofness and air permeability.
Patent Document 2 describes a ventable plug to be attached to an electric motor case for automotive power windows. In a sheet joining portion of this cylindrical plug body, an air-permeable sheet is formed so as to close the inside of the plug body, so that water is prevented from entering the motor case, and the function of an air-permeable membrane allows ventilation between the inside and outside of the case. In Patent Document 2, the plug body portion is formed in an outer portion of the plug, and is formed so as to protrude above a plane including the air-permeable membrane. Thus, during the operation of attaching the ventable plug to the motor case, the protruding part serves as a protective dike, and therefore, a hand does not often make contact with the air-permeable membrane. This contributes to the prevention of breakage of the air-permeable membrane.